In response to a request by the author, Charles W. Trigg has contributed the following discussion of coffee making:
Various Aspects of Scientific Coffee Brewing
Before converting it into the beverage form, coffee must be carefully selected and blended, and skillfully roasted, in order thus far to assure obtaining a maximum efficiency of results. No matter how accurately all this be done, improper brewing of the roasted bean will nullify the previous efforts and spoil the drink; for roasted coffee is a delicate material, very susceptible to deterioration and of doubtful worth as the source of a beverage unless properly handled.
There probably never was produced a drink which so fits into the exacting desires of the human appetite as does coffee. Properly prepared, it is a delightful beverage: but incorrectly made, it becomes an imposition upon the palates of mankind. Sensitive though coffee is to improper manipulation, the best procedure for brewing it is also the easiest. Cheap coffee well made excels good coffee poorly made.
Constituent Concepts. The roasting of green coffee causes an alteration in the constitution of its constituents, with the result that some of the compounds present therein which were originally water-soluble are rendered insoluble, and some which were insoluble are converted into soluble ones. A portion of the original caffein content is lost by sublimation. The aromatic conglomerate, caffeol, is formed, and a considerable quantity of gas is produced, a portion of which, developing pressure in the cells of the beans, pops, or swells, them so as to increase the size of each individual bean. The constituents which are water-soluble after the torrefaction may be generally classified as heavy extractives and light aromatic materials. The percentages and nature of these materials in the roasted coffee will vary with the type of coffee and with the roast which it is given. In general, and in particular for purposes of comparison of methods of brewing, they may be considered to be the same and to occur in about the same proportions in all coffees.
The heavy extractives are caffein, mineral matter, proteins, caramel and sugars, "caffetannic acid", and various organic materials of uncertain composition. Some fat will also be found in the average coffee brew, being present not by virtue of being water soluble, but because it has been melted from the bean by the hot water and carried along with the solution.
The caffein furnishes the stimulation for which coffee is generally consumed. It has only a slightly bitter taste, and because of the relatively small percentage in which it is present in a cup of coffee, does not contribute to the cup value. The mineral matter, together with certain decomposition and hydrolysis products of crude fiber and chlorogenic acid, contribute toward the astringency or bitterness of the cup. The proteins are present in such small quantity that their only rôle is to raise somewhat the almost negligible food value of a coffee infusion. The body, or what might be called the licorice-like character of coffee, is due to the presence of bodies of a glucosidic nature and to caramel.
As has been previously pointed out[382], the term "caffetannic acid" is a misnomer; for the substances which are called by this name are in all probability mainly coffalic and chlorogenic acids. Neither is a true tannin, and they evince but few of the characteristic reactions of tannic acid. Some neutral coffees will show as high a "caffetannic acid" content as other acid-charactered ones. Careful work by Warnier[383] showed the actual acidities of some East Indian coffees to vary from 0.013 to 0.033 percent. These figures may be taken as reliable examples of the true acid content of coffee, and though they seem very low, it is not at all incomprehensible that the acids which they indicate produce the acidity in a cup of coffee. They probably are mainly volatile organic acids together with other acidic-natured products of roasting.
